1. Technical Field
The present invention relates to an oil-impregnated sintered bearing and to a production method therefor. In particular, the present invention relates to an oil-impregnated sintered bearing for an electric motor, which may be mounted in an automobile or the like and is suitably used in cold climates, and relates to a production method therefore.
2. Background Art
Oil-impregnated sintered bearings are made such that a bearing body is formed of a porous sintered compact and a lubricating oil is impregnated into pores of the sintered compact. Therefore, the oil-impregnated sintered bearings can be used for a long time without providing additional oil. For this advantage, the oil-impregnated sintered bearings are applied in various bearing devices and have also been used as bearings for various electric motors in manufacturing of automobiles. The electric motors are arranged in interior space of automobiles, and therefore, it is undesirable if noise, so-called “squealing noises” are generated by metallic contact between a shaft and an inner circumferential surface of a bearing during sliding. In this regard, various means for preventing generation of the squealing noises have been developed.
According to the inventions disclosed in Japanese Patent Applications of Laid-open Nos. 2003-120674 (patent document 1) and 2005-082867 (patent document 2), the following oil-impregnated sintered bearings are provided. These oil-impregnated sintered bearings do not produce the squealing noises even when used in cold climates that reach a temperature of, for example, 20° C. below zero or 30° C. below zero. These oil-impregnated sintered bearings are formed with conflicting characteristics in which an open porosity is high but fluid permeability is low, thereby preventing generation of the squealing noises. The fluid permeability affects leakage characteristics of lubricating oil and oil pressure of a sliding surface. Therefore, an oil-impregnated sintered bearing with high fluid permeability tends to produce the squealing noises during sliding in cold climates. On the other hand, when the fluid permeability is decreased by increasing the density of an oil-impregnated sintered bearing, the number of the pores is decreased. As a result, the open porosity is decreased, and oil-retaining characteristics are degraded. Accordingly, when the open porosity is increased in order to improve the oil-retaining characteristics, the fluid permeability is increased. Thus, the fluid permeability and the open porosity are in a trade-off relationship, and it is difficult to increase the open porosity (oil-retaining characteristics) and to decrease the fluid permeability at the same time.
In regard to this problem, in the inventions disclosed in the patent documents 1 and 2, by using a porous reduced iron powder as an iron powder, an enormous number of microscopic pores are arranged in an iron phase of the oil-impregnated sintered bearing, whereby the open porosity is increased. Thus, while the fluid permeability is maintained to be low, the open porosity is increased, and the problem of the conflicting characteristics of the fluid permeability and the open porosity is overcome. Accordingly, oil-impregnated sintered bearings that do not produce the squealing noises even when used in cold climates are provided.
Specifically, the oil-impregnated sintered bearing disclosed in the patent document 1 is made of a sintered alloy having a cross sectional structure in which Cu alloy phase containing Sn and P and ferrite phase are mixed at approximately equal amounts by area ratio. The sintered alloy includes not more than 0.7 mass % of graphite particles and includes pores with an open porosity of 20 to 30%. The pores are impregnated with a synthetic oil with a kinetic viscosity of 61.2 to 74.8 mm2/s (cSt) at 40° C. The oil-impregnated sintered bearing is sized and has an inner circumferential surface at which iron portion is exposed at 2 to 6% by area ratio. The oil-impregnated sintered bearing has a fluid permeability of 6 to 50×10−11 cm2.
On the other hand, the oil-impregnated sintered bearing disclosed in the patent document 2 is produced by compacting and sintering a mixed powder of an iron powder and one of a copper powder and a copper alloy powder. In this method, a part of the amount or the entirety of the iron powder is a porous iron powder. The porous iron powder is made of particles which include numerous microscopic pores from the surface to the inside thereof and thereby have spongelike appearances. The porous iron powder particles have sizes of not more than 177 μm and pass through a sieve of 80 mesh. The porous iron powder has a specific surface area of 110 to 500 m2/kg that is measured by a gas adsorption method.
The oil-impregnated sintered bearings disclosed in the patent documents 1 and 2 are suitably used as bearings for blowers of car air-conditioners, for example. In contrast, when these oil-impregnated sintered bearings are used as bearings of window regulator motors so as to open and close the windows of automobiles, they produce squealing noises even at ordinary temperatures. In regard to the car air-conditioner, when once it is turned on, it may be used continuously for a while. Therefore, after the car air-conditioner is started, sufficient amount of lubricating oil is supplied, and an oil film is reliably formed between a shaft and an inner circumferential surface of the bearing. On the other hand, the window is operated for only a short time and is intermittently used. Therefore, the window is stopped before sufficient amount of lubricating oil is supplied and an oil film is reliably formed between a shaft and an inner circumferential surface of the bearing. Accordingly, the oil film is not sufficiently formed between the shaft and the inner circumferential surface of the bearing at any time.